1. Field of the Invention
This invention relates to an impedance element which exhibits a sufficiently high impedance in a predetermined frequency band but shows a sufficiently low impedance outside of the predetermined frequency band, and a band-rejection filter employing the impedance element.
2. Description of the Prior Art
Heretofore, there have been proposed a variety of impedance elements or circuits which show a high impedance in a predetermined frequency band but exhibit a low impedance outside thereof. By inserting such an impedance element or circuit, for example, in series, in a transmission line, a signal supplied to the input side of the transmission line is derived at the output side thereof while being rejected over the predetermined frequency band in which the impedance element or circuit exhibits a high impedance. Namely, a band-rejection filter is provided. The band-rejection filter characteristic of the band-rejection filter thus formed is dependent upon the frequency-impedance characteristic of the impedance element or circuit used.
With the conventional types of impedance elements or circuits, in the case where the predetermined frequency band in which they show a high impedance is selected to lie within a frequency band such as the VHF or UHF band, it is relatively difficult to make the width of the predetermined frequency band narrow and to obtain a sharp or abrupt frequency-impedance characteristic. Further, it is also relatively difficult to make sufficiently large the ratio of the impedance exhibited in the predetermined frequency band to the impedance in the other frequency bands. Moreover, in the case of forming an impedance element exhibiting such a frequency-impedance characteristic which provides a relatively narrow width of the predetermined frequency band for high impedance, and is sharp or abrupt, there is a disadvantage that the impedance element is large in size.
As a result of this, in a band-rejection filter constructed with the conventional impedance element, it is particularly difficult to obtain in the VHF or UHF band such a band-rejection filter characteristic which provides a relatively narrow band-width of a predetermined rejection band and is sharp. Further, in the conventional band-rejection filter, it is relatively difficult to obtain such a band-rejection filter characteristic that attenuation in a predetermined rejection band is sufficiently larger than attenuation in the other frequency bands. Moreover, the conventional band-rejection filter has a defect of bulkiness.